synthesis of acetylenes, allenes and cumulenes || copper halide-catalysed oxidative coupling of...

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E:/Archive files/4188-Brandsma/Printer-Files/4188-Chapter-15.3d

15Copper Halide-Catalysed Oxidative

Coupling of Acetylenes

15.1 METHODS, SCOPE AND LIMITATIONS

The original procedure [1] using pre-formed copper acetylide has evolved to a

number of variants in which the acetylene is oxidised with oxygen or air in the

presence of catalytic amounts of copper(I) halide. The overall equation is:

Instead of introducing oxygen, copper(II) salts can be used in stochiometri-

cal amounts. Copper seems to have an unique role. Salts of cobalt and iron are

not capable of catalysing the coupling. The various coupling methods have

been reviewed [2,3].

A wide variety of acetylenic compounds have been oxidatively ‘dimerised’.

Several examples are mentioned in the reviews. Especially in procedures on a

small scale relatively large amounts of copper salts have been used, often much

more than stochiometrically required.

Depending on the acidity of the ethynyl proton in RC�CH, the nature of

R and other factors, the oxidative coupling may be carried out in an organic

solvent or in aqueous medium, following the general procedures a and b.

a. Introduction of oxygen into a vigorously agitated mixture of a 1-alkyne,

RC�CH, and an organic solvent containing a catalytic amount (up to � 10

mol%) of copper(I) halide [4]. In order to solubilise the copper salt,

pyridine or TMEDA (formation of the bidentate complex) is used. The

amine also facilitates the (reversible) proton removal from the 1-alkyne.

Pyridine can be used as solvent, though many chemists will prefer the non-

smelling DMF. The easily removable acetone is particularly attractive

[4,5]. Aryl- or hetaryl-acetylenes, (Het)ArC�CH, enynes, R1CH¼CHC�

CH, diynes, R1C�CC�CH, and triethylsilylacetylene, Et3SiC�CH, react

281

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very smoothly under the influence of CuX � pyridine or CuX �TMEDA

complexes and yields are generally excellent. Also acetylenic tertiary alco-

hols, HC�CC(R1)(R2)OH, react satisfactorily, but for primary alcohols

and the acetylenic amines HC�CC(R1)(R2)NH2 the aqueous procedure

b is more suitable. Acetylenes without conjugation, e.g. aliphatic 1-alkynes

[7], and also 2-ethynyl-1-methylpyrrole [6], react sluggishly, but addition of

the more strongly basic 1,8-diaza[5.4.0]bicycloundec-7-ene (DBU) greatly

facilitates their oxidative coupling [6,7].

b. Reaction of oxygen with a mixture of the acetylene and an aqueous solu-

tion of ammonium chloride containing copper(I) halide. The amount of

copper halide is generally much larger than that used in method a. This

method can be used for the oxidative couplings of primary (HC�

C(CH2)nOH), secondary (HC�CCH(R)OH) and tertiary alcohols [8],

HC�CC(R1)(R2)OH), acetylenic carboxylic acids [8] (e.g. 4-pentynoic

acid, HC�CCH2CH2COOH), and amines [9] (e.g. N-t-butyl-2-propyn-1-

amine, HC�CCH2NH-t-Bu). Successful conversion of butenyne, HC�

CCH¼CH2, into 1,7-octadien-3,5-diyne, H2C¼CHC�CC�CCH¼CH2,

has been achieved by using diethyl ether as a co-solvent [8]. Oxidative

couplings of methyl 10-undecynoate, HC�C(CH2)8COOMe, 1-penten-4-yne,

H2C¼CHCH2C�CH, 1-alkyn-o-ols, HC�C(CH2)nOH, 5-hexyn-2-one,

HC�CCH2CH2COMe, 2-penten-4-yn-1-ol, HC�CCH¼ CHCH2OH,

have been carried out in water or alcohol-water mixtures [10–14].

As the oxidative couplings proceed smoothly over a wide pH range, acid- as

well as base-sensitive acetylenes can be dimerised with satisfactory results.

Acetylenic amines can be coupled as their HCl salts.

A number of heterosubstituted acetylenes do not give the coupling products,

due to the presence of strongly complexing groups [15] in ethynylphosphines,

HC�CPR2, C-heteroatom cleavage [15] in ethynyl(trialkyl)stannanes,

HC�CSnR3, and led analogues, HC�CPbR3, or reactions involving strongly

activated triple bonds [16], e.g. HC�COEt (however, cf. [22]). Several attempts

to couple methyl propiolate, HC�CCOOMe, and 2-propyn-1-amine,

HC�CCH2NH2, have failed [17].

Under basic conditions the order of the reaction rates of oxidative couplings

is found to be parallel with the expected order of acidities of the acetylenes

[18,19]. Thus, enynes RCH¼CHC�CH, and diynes, RC�CC�CH, reacted

faster than do acetylenes with a non-conjugated triple bond.

Discussions on the various mechanistic proposals can be found in the

reviews [2,3,20].

Our rather extensive experience with oxidative couplings shows, that

none of the various solvents gives generally satisfactory results. Many acety-

lenic derivatives can be successfully coupled in pyridine, which is a good

282 15. COPPER HALIDE-CATALYSED OXIDATIVE . . .

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complexing agent for copper compounds. However, some couplings of acet-

ylenic compounds containing polar groups such as OH and C¼O do not give

optimal results in pyridine, while the work-up may be laborious. Application

of DMF, acetone or other volatile organic solvents provides complementary

possibilities for oxidative couplings. Water is a suitable solvent for couplings

of the lower acetylenic alcohols such as HC�CCH2OH. Also some more

lipophilic acetylenes have been successfully coupled using a two-phase

system of water and an organic solvent [8]. In a number of cases the coupling

may stop or proceed very sluggishly due to formation of a slightly soluble

copper compound. The only general advice that we can give is to try another

solvent. Aryl- or hetarylacetylenes (except 2-ethynyl-1-methylpyrrole), enynes

(R1CH¼CHC�CH) and diynes (R1C�CC�CH) very readily ‘dimerise’ and

the choice of the solvent is determined only by considerations involving the

ease of the work-up.

15.2 EXPERIMENTAL SECTION

For a summary of the various experimental procedures and some procedures

from literature see Table 15.1.

15.2.1 Oxidative coupling of propargyl alcohol in aqueous medium

Scale: 0.50 molar; Apparatus: Figure 1.9, 500 ml, with long gas inlet tube

15.2.1.1 Procedure

After completely replacing the air in the flask by oxygen, the rate of introduction

of oxygen is adjusted at� 100 ml/min. The flask is charged with 100 ml of a cold

(� 5 �C) saturated aqeous solution of ammonium chloride and 5 g of finely

powdered copper(I) chloride (technical grade may be used). After addition of

4 g of freshly distilled propargyl alcohol (under gentle stirring) at rt, the colour of

the mixture (first green) becomes very light. The rate of stirring is increased in

order to effect intensive mixing of the solution with oxygen (high turbulence).

The mixture is brought at 30 �C, after which the temperature gradually rises to

above 35 �C. By occasional cooling the temperature is maintained in the region

of 40 �C. When the green colour begins to return, a second portion of � 4 g of

propargyl alcohol is added (stirring is temporarily stopped). The remainder

15.2 EXPERIMENTAL SECTION 283

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of the 0.50 mol is added in 4-g portions over � 1.5 h. Stirring (at 40 �C)

after addition of the last portion is continued for an additional period of 30 to

45 min. The green suspension is cooled to rt, after which six to eight extractions

with a 1:3 mixture of THF and Et2O are carried out (first twice with 100-ml

portions, for the other extractions 50-ml portions). The light-brown extracts are

combined and stirred during 30 min with 50 g of anhydrous potassium carbo-

nate. After filtration and thorough rinsing of the drying agent with the

Et2O–THF mixture, the solution is concentrated under reduced pressure. The

remaining light brown solid is powdered (mortar) and subsequently heated at

50 �C (with occasional manual swirling) in a vacuum of <1 Torr in order to

remove the last traces of solvent. The yield of pure product is greater than 85%.

Table 15.1

Copper(I) chloride-catalysed oxidative coupling of acetylenic compounds*

Acetylenic compound Reaction conditions Refs. and notes

HC�CCH2OH NH4C1, H2O, �40

HC�CCH2OH TMEDA, acetone, 40

HC�CCH(Me)OH TMEDA, acetone, 45–50

HC¼CCMe2OH TMEDA, acetone, 45–50

HC�CCH2CH2OH TMEDA, acetone, 45–50

HC�CCH2OMe TMEDA, acetone, 40–50

(Z)-HC–CCH�CHOMe TMEDA, acetone, 45–50

HC�CAr TMEDA, acetone, 45–50

HC�CSiEt3 TMEDA, acetone, air, rt, 6 h Ref. 5; excellent yield

HC�CO–t-Bu TMEDA, acetone, rt, 1 h Ref. 22; high yield

HC�CCH(OEt)2 TMEDA, DMF, 45-50

HC�CCH2SEt TMEDA, DMF, 40

2-Ethynylpyridine TMEDA, MeOCH2CH2OMe,

35 pyridine, rt

Ref. 21; 1 h, 79% yield

HC�C(CH2)4OH pyridine, 40–45

HC�CEt DBU, pyridine, 30–35

HC�C–t-Bu DBU, pyridine, 35–40

HC�CSiMe3 DBU, pyridine, 35–40

2-Ethynyl-1-methylpyrrole DBU, pyridine, 35–40

HC�C(CH2)2COOH NH4C1, acetone, H2O, 0 Ref. 8; quant. yield

HC�CCMe2NH2 �HC1 NH4C1, H2O, � 50

HC�CCH2NH–t-Bu NH4C1, 2 M HCI, 55, 6 h Ref. 9; >100 mol % CuCl

HC�CCH2CH¼CH2 NH4Cl, HCI (diluted) Ref. 11; excellent yield

HC�C(CH2)3OH NH4C1, HCI (diluted) Ref. 11; 100% yield

HC�CC�CSiEt3 Same conditions, 1.5 h Ref. 5; excellent yield

HC�CSEt NH4OH (conc.), MeOH,

rt, 1 h

Ref. 16

*Temperatures in �C.


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15.2.2 Coupling of 3-butyn-2-ol using copper(I) chloride �TMEDAin acetone

Scale: 0.50 molar; for the equipment see exp. 15.2.1

15.2.2.1 Procedure [4]

After filling the flask with oxygen, 90 ml of acetone, 3.2 g of TMEDA, 3.2 g of

finely powdered copper(I) chloride and � 5 g of of 3-butyn-2-ol (commercially

available) are introduced with intervals of a few seconds and with stirring at a

moderate rate. The flow of oxygen is adjusted at � 100 ml/min and very vigor-

ous stirring is started (intensive mixing of the solution with oxygen). The

mixture is heated to 45 �C. The temperature of the green-blue solution rises

to over 50 �C within a few min. Occasional cooling is applied to keep the

temperature between 45 and 50 �C. When the temperature has begun to drop

(without cooling) and the colour of the solution has become darker, a second

portion of 5 g of the acetylenic alcohol is added. The remaining amount is

introduced in 5 g-portions over approximately half an hour. When after addi-

tion of the last portion the temperature begins to drop, the reaction mixture is

heated in a bath at 45 �C. After an additional half an hour most of the acetone

is removed under reduced pressure (rotary evaporator). The residue is treated

with 100 ml of a saturated solution of ammonium chloride containing some

ammonia, after which five extractions with Et2O are carried out. The combined

extracts (washing is not carried out) are dried over anhydrous potassium car-

bonate. After concentration in vacuo (in the last stage a high vacuum is

applied) a viscous light-brown oil remains, which slowly solidifies upon stand-

ing at rt. The 1H-NMR-spectrum (4.45 and 1.42 ppm) indicates that the

product has a satisfactory purity. The yield is almost quantitative.

2-Methyl-3-butyn-2-ol, HC�CCMe2OH (0.15 mol, added in one portion), is

converted into 2,7-dimethyl-3,5-octadiyn-2,7-diol by a similar procedure, using

70 ml of acetone, 1.2 g of CuCl, 1.2 g of TMEDA. The work-up is carried out

by adding aqueous NH4Cl (20 g in 500 ml) containing a small amount of

ammonia. The solid diol is obtained in almost 100% yield.

For the oxidative coupling of 3-butyn-1-ol, HC�C(CH2)2OH, (0.15 mol)

80 ml of acetone and the same amounts of CuCl and TMEDA are used.

After the reaction has finished, 5 ml of water is added, and the acetone is

removed under reduced pressure. The greenish residue is extracted five times

with Et2O. The extract is dried over potassium carbonate, which adsorbs


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dissolved copper compounds. After thorough removal of the ether under

reduced pressure, 3,5-octadiyn-1,8-diol remains as a viscous liquid (yield

� 100%) slowly solidifying at rt.

Similar procedures (on a 0.10–0.15 molar scale) are applicable for 2-propyn-

1-ol, HC�CCH2OH, 3-methoxy-1-propyne, HC�CCH2OMe, (Z)-1-methoxy-

1-buten-3-yne, HC�CCH¼CHOMe, 2-t-butyl-1-buten-3-yne, HC�

CC(t-Bu)¼CH2, ethynylbenzene, PhC�CH, 2-ethynylthiophene and 2-ethynyl-

furan. In procedures on a larger scale the substrate is added portionwise.

In the cases of ethynyl(trimethyl)silane, Me3SiC�CH, and 3-ethylthio-1-

propyne, HC�CCH2SEt, the reaction stops in an early stage. With 3,3-

diethoxy-1-propyne, HC�CCH(OEt)2, the reaction at � 45 �C proceeds

rather slowly [17].

15.2.3 Oxidative coupling of 3,3-diethoxy-1-propyneusing CuCl �TMEDA in DMF

Scale: 0.15 molar; Apparatus: Figure 1.9, 500 ml, oxygen is introduced at a rate

of � 100 ml/min.

15.2.3.1 Procedure

After completely replacing the air by ogygen, 110 ml of DMF, 1.5 g of

freshly powdered copper(I) chloride, 5 g of TMEDA and 0.15 mol of

freshly distilled 3,3-diethoxy-1-propyne, are introduced with intervals of a

few seconds. A greyish suspension is formed. After starting very vigorous

stirring, the temperature rises to above 40 �C within a few minutes.

Occasional cooling may be necessary. After the exothermic reaction has sub-

sided (dropping of the temperature) the mixture is stirred for an additional

20 min at � 40 �C. The bluish-green solution is then treated with 500 ml of

water and five extractions with a 1:1 mixture of Et2O and pentane are

carried out. The combined extracts are washed with water and dried over

anhydrous potassium carbonate. After removal of the solvent under reduced

pressure pure 1,1,6,6-tetraethoxy-2,4-hexadiyne remains as an almost colour-

less liquid. Yield � 95%.

Ethyl propargyl sulphide, HC�CCH2SEt, is converted (� 85% yield) into

1,6-bis(ethylthio)2,4-hexadiyne by a similar procedure.


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15.2.4 Oxidative coupling of 5-hexyn-1-ol in pyridine

Scale: 0.20 molar; Apparatus: Figure 1.9, 500-ml; oxygen is introduced at a rate

of � 100 ml/min.

15.2.4.1 Procedure

5-Hexyn-1-ol (0.20 mol) is dissolved in 110 ml of pyridine and 1 g of finely

powdered copper(I) chloride is added with stirring at a moderate rate. The

green solution is then stirred vigorously and the temperature rises within

15 min to � 40 �C. The temperature of the mixture is kept between 40 and

45 �C by occasional cooling. After the exothermic reaction has subsided and

the temperature has begun to drop, the mixture is stirred for another half an

hour at � 40 �C, during which period the colour gradually becomes dark-

green. The greater part of the pyridine is removed on the rotary evaporator.

The remaining liquid is treated with a sufficient amount of cold (0 �C) dilute

aqueous hydrochloric acid (4 N). The solution is extracted four times with

Et2O. The combined extracts are washed with water and subsequently dried

over magnesium sulphate. After removal of the solvent in vacuo small

amounts of the starting compound are distilled off in a high vacuum. The

residue (yield � 85%) is almost pure 5,7-dodecadiyn-1,12-diol. It solidifies

after cooling to rt.

15.2.5 Oxidative coupling of 2-ethynylpyridine

Scale: 0.20 molar; Apparatus: Figure 1.9, 500 ml, without dropping funnel;

oxygen is introduced at a rate of � 100 ml/min.

15.2.5.1 Procedure

A mixture of 0.20 mol of 2-ethynylyridine, 2.0 g of finely powdered copper(I)

chloride and 100 ml of pyridine is vigorously stirred, while keeping the temp-

erature of the mixture between 15 and 20 �C (if the temperature is allowed to


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rise above 30 �C, much brown tarry material is formed). After 1.5 h the dark

mixture is poured into 300 ml of water. The mixture is extracted four times

with small portions of chloroform. The combined extracts are dried over

anhydrous potassium carbonate, after which the solvent is removed

under reduced pressure. The last traces of solvent are removed in an oil-

pump vacuum (<0.5 Torr). The remaining solid (mp 119–120 �C) is pure

di(2-pyridyl)butadiyne. Yield � 80%.

Oxidative couplings of phenylacetylene, 2-ethynylfuran 3-ethynylpyridine,

2-ethynylthiophene, 5,5-dimethyl-1,3-hexadiyne, t-BuC�CC�CH, and N,N-

diethyl-2-propyn-1-amine, HC�CCH2NEt2, are successfully carried out by

similar procedures at � 35 �C. Stirring and introduction of oxygen are stopped

when the temperature begins to drop fast and the colour of the mixture has

become very dark-green or greenish brown. Yields are generally excellent.

15.2.6 Oxidative coupling of 1-butyne catalysed by CuCl and1,8-diaza[5.4.0]bicycloundec-7-ene (DBU)

Scale: 0.50 molar;Apparatus: Figure 1.9, 1-litre. The outlet is connected to a cold

trap (–78 �C); oxygen is passed through the flask at a rate of 100–150 ml/min.

All connections are made gas-tight.

15.2.6.1 Procedure

After the air in the flask has been completely replaced by oxygen, 250 ml of

pyridine, 2 g of finely powdered copper(I) chloride and 3 ml of DBU are

introduced. The mixture is cooled to � 10 �C and 0.50 mol of 1-butyne (liqui-

fied in a cold trap, � –70 �C) is added. Vigorous stirring with introduction of

oxygen is started. The temperature of the mixture gradually rises, but is kept

between 25 and 30 �C by occasional cooling in a bath at –10 �C. When the

reaction has subsided, the contents of the cold trap are returned into the flask

(usually not more than a few ml). Stirring at 35 �C is then continued for an

additional period of 30 min. The dark-green solution is poured into 1 litre of

ice water, after which ten extractions with small (first portion 70 ml, subse-

quently � 30 ml) portions of pentane are carried out. The combined extracts

are washed with cold dilute hydrochloric acid and subsequently dried over

magnesium sulphate. Most of the pentane is distilled off at normal pressure

through an efficient column. The remaining liquid is distilled in vacuo.

3,5-Octadiyne, bp 40 �C/15 Torr, is obtained in greater than 75% yield.


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15.2.7 Oxidative coupling of 2-ethynyl-1-methylpyrrolecatalysed by CuCl and DBU

Scale: 0.05 molar; Apparatus: Figure 1.9, 250 ml; magnetic stirring; oxygen

flow: � 100 ml/min.

15.2.7.1 Procedure [6]

A mixture of 0.05 mol of 2-ethynyl-1-methylpyrrole, 50 ml of pyridine, 1 g of

finely powdered copper(I)chloride and 2 g of DBU is vigorously stirred. The

temperature of the mixture, initially 20 �C, rises to above 35 �C, but is kept

between 35 and 40 �C by occasional cooling. After the temperature has begun

to drop, stirring is continued for an additional half an hour at 30–35 �C. The

mixture is poured into 500 ml of water, after which five extractions with Et2O

are carried out. The combined extracts are dried over anhydrous potassium

carbonate and subsequently concentrated in vacuo . The last traces of pyridine

are removed in a high vacuum of <0.5 Torr 1-methyl-2-[4-(1-methyl-1H-2-yl)

1,3-butadiynyl]1-1H-pyrrole remains as a light-brown solid. Yield >80%.

In the absence of DBU no reaction takes place.

t-Butylacetylene is ‘dimerised’ with an excellent yield by a similar procedure

to give white crystals [7].

15.2.8 Oxidative coupling of ethynyl(trimethyl)silane

Scale: 0.10 molar; Apparatus: Figure 1.9, 500 ml; oxygen is introduced at a rate

of � 100 ml/min.

15.2.8.1 Procedure

After completely replacing the air in the flask by oxygen, 50 ml of DMF, 2 ml

of pyridine, 1 g of finely powdered copper(I) chloride and 0.10 mol of tri-

methylsilylacetylene are placed in the flask. Vigorous stirring is started, causing


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the temperature to rise within a few minutes from 20 to � 40 �C. Occasional

cooling is necessary to keep the temperature between 35 and 40 �C. After the

temperature has begun to drop (from 40 �C) stirring is continued until the

colour of the mixture has become brown. The mixture is poured into 500 ml

of ice water, after which four extractions with small portions of pentane are

carried out. The combined organic solutions are washed with cold (0 �C) 2 M

hydrochloric acid in order to remove traces of pyridine and subsequently dried

over magnesium sulphate. After evaporation of the pentane in vacuo bis(tri-

methylsilyl)butadiyne remains as light-brown crystals. Yield � 80%.

Using CuCl in pyridine or CuCl �TMEDA in acetone, poor results are

obtained.

15.2.9 Oxidative coupling of the HCl–salt of2-methyl-3-butyn-2-amine

Scale: 0.20 molar; Apparatus: Figure 1.9, 250 ml; oxygen is introduced at a rate

of � 100 ml/min.

15.2.9.1 Procedure

Concentrated, aqueous hydrochloric acid (36%) is added dropwise at 0 �C to a

mixture of 0.20 mol of 2-methyl-3-butyn-2-amine and 150 ml of a saturated

aqueous solution of ammonium chloride until the pH has become 6. Finely

powdered copper(I) chloride (8 g) is introduced, after which the mixture is

warmed to 45 �C. A yellowish solution is formed. The flask is insulated

in cotton wool and very vigorous stirring is started. The temperature rises

within half an hour to above 50 �C. Stirring is stopped when the temperature

has dropped to 30 �C, then 30 ml of a concentrated aqueous solution of

ammonia and 100 ml of water is added to the light green suspension. The

blue solution is extracted seven times with Et2O. The ethereal solutions are

dried (without washing) over anhydrous potassium carbonate and subse-

quently concentrated under reduced pressure. Pure 2,7-dimethyl-3,5-octa-

diyne-2,7-diamine remains as a light-brown solid. Yield � 85%.

Under similar conditions 2-propynylamine, HC�CCH2NH2, gives an amor-

phous red solid [17].


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synthesis of acetylenes, allenes and cumulenes || copper halide-catalysed oxidative coupling of...

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